1. Field of the Invention
The present invention relates in general to semiconductor memory devices such as dynamic random access memories, and more particularly to a semiconductor memory device for preventing a metal bridge from being generated between adjacent metal lines because of an increase in memory capacity.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a circuit diagram illustrating a memory cell array in a conventional semiconductor memory device. As shown in this drawing, a memory cell array block comprises a plurality of word lines 112, a plurality of bit lines and a plurality of memory cells, each of which includes an NMOS transistor and a capacitor.
In FIG. 1, the word lines are connected directly to a decoder 110 and each of them comprises polysilicon. However, the above-mentioned conventional semiconductor memory device has a disadvantage in that data stored in the memory cells cannot rapidly be accessed, particularly in the case where the memory device has a large capacity. In order to overcome such a problem, there has been proposed a method of forming a metal strap at a predetermined distance from each of the word lines as shown in FIG. 2 (Vpp represents the voltage from a high voltage generator). However, the above-mentioned method has a disadvantage in that the metal cannot compete with a tight pitch between adjacent ones of the word lines because chip density is gradually increased according to the design rule of sub-micron. To solve the problem with the above-mentioned method, there has recently been developed a method of adding sub-decoders to the construction of FIG. 2.
Referring to FIG. 3, there is shown a circuit diagram illustrating a memory cell array in a conventional semiconductor memory device in which sub-decoders are formed. In this construction of FIG. 3, enable and disable metal lines must be provided to allow a main row decoder to enable and disable the sub-decoders. Generally, in a semiconductor memory device employing sub-decoders, one metal line is formed for two word lines. In this case, a pitch between adjacent metal lines becomes narrower due to a high integration degree of the semiconductor memory device, thereby significantly increasing the probability that a metal bridge will be generated in the manufacturing process. Such a metal bridge results in the formation of a short circuit between supply and ground voltage sources, thereby making it impossible to normally operate the semiconductor memory device. Furthermore, it is very hard to repair the semiconductor memory device.